Regardless, the goal of reduced speed in a school zone is to protect unaware children/increased slow traffic, so prior to any activities going on it shouldn't matter. The other options are however that it was a construction Zone, which would require signs, or they wanted people to get used to slowing down around that area, which would certainly require signs since they obviously wouldn't rely on the inference of every driver...

More concisely, if there weren't signs OP shouldn't be subject to punishment.

Just because there's no sign posted, doesn't mean there's no speed limit. Here, most cities have a sign that says the speed limit is x unless otherwise posted. That way, even if there is no sign, you can't go a ridiculous speed and use that as an excuse

55- yes, but school's usually have reduced speed limit signs because of the unaware children. My school has signs that limit the speed to 20 mph. The signs have lights and are only active when the lights are flashing (during school days and hours)

Hey, your mental punch really hurts:)
My little brother sustained a head injury in his second grade while getting into his school. The car driver blamed the city council, because weeks ago there was no active school. Just a building. He didn't even apologize.